1. Field of the Invention
This invention relates to video replay stores.
2. Description of the Prior Art
A digital video tape recorder (DVTR) necessarily requires a video replay store to form the interface between the DVTR, which supplies off-tape asynchronous data streams, and television apparatus synchronized to, for example, a 625-line 50 fields per second or 525-line 60 fields per second television system. The primary function of the video replay store is picture assembly, but it is usual to include other functions, such as data de-shuffling, time-base correction, picture synchronization and error flag management.
Particularly in the case of a multi-channel DVTR, the hardware is somewhat complex because of the number of field stores required, and the complexity is increased still further if the DVTR is to be operable in so-called stund modes, that is, at forward and reverse speeds different from the normal reproduction speed, and including the still mode. In such a DVTR some problems arise with the replay store.
Firstly, in the head drum of a DVTR it is physically impossible to align the heads so that each channel reads data at exactly the same time. Even if it were possible, the exact relationship between data of different channels could not be maintained due to, for example, mechanical instability. Therefore it must be assumed that each channel completes the writing of a field of data in the video replay store independently of the others. At this point the channel will then request a store change from the controller. With all the channels asynchronously requesting store changes, and since the stores of all the channels must change together, because eventually all the channels will be combined to form a single output, some means is necessary to combine these store change requests.
Secondly, even under normal operational conditions, it is perfectly possible for one or more channels to fail. This can occur due to head clog, drop-out, head damage, high error rates or circuitry failure. Under these conditions it is required that the remaining channels continue to operate correctly, even to the extreme of operating on one channel alone. In effect this means that signals from the stores, and in particular request for store change from any one store, cannot be relied upon; and that so long as at least one store is operating correctly, store changes should continue.
Thirdly, requests to read from or write in a given store must be correctly synchronized with the store operation, that is with the memory control signals, in order to prevent memory crashes. Synchronization is usually achieved by synchronizing the read enable with the read circuitry of the store, and the write enable with the write circuitry. As a result it is possible, for example, for the write enable to be synchronized and hence request the store to start writing before the read enable has been synchronized to stop the store reading. There could therefore be contention between the reading and writing of a store.